function loadJS( url, callback ){

    var script = document.createElement('script'),

        fn = callback || function(){};

    script.type = 'text/javascript';



    //IE

    if(script.readyState){

        script.onreadystatechange = function(){

            if( script.readyState == 'loaded' || script.readyState == 'complete' ){

                script.onreadystatechange = null;

                fn();

            }

        };

    }else{

        //其他浏览器

        script.onload = function(){

            fn();

        };

    }

    script.src = url;

    document.getElementsByTagName('head')[0].appendChild(script);

}
